<!DOCTYPE html>
<head>
  <style>
    div {
      position: absolute;
      left: 100px;
      top: 100px;
      width: 50vw;
      height: 50vw;
      background-color: black;
    }
  </style>
  <script>
    if (window.testRunner) {
      addEventListener('load', function() {
        // Zoom in three times. Ensure the image scales just as a div with
        // an explicit vw size would.
        testRunner.zoomPageIn();
        testRunner.zoomPageIn();
        testRunner.zoomPageIn();
      });
    }
  </script>
</head>
<body>
  <div></div>
</body>
